The present invention relates to a two-dimensional radiation detector for use in medical, industrial as well as atomic power fields and the like to detect radiation.
In the conventional art, the radiation detector of this type contains a radiation sensitive radiation converting layer (semiconductor layer) that converts radiation information into electric charge information and an active matrix substrate that reads out the converted electric charge.
In the conventional radiation detector, the electric charge information obtained at the radiation converting layer is read out by the active matrix substrate, wherein the electric charge information is processed by an amplifier and the like, and output externally as radiation detection signals. The radiation detection signals obtained in such a way are used to generate a fluorographic image or the like.
However, this type of conventional technology has the following problems.
A phenomenon of fluctuation in the detection sensitivity of the radiation converting layer occurs. A phenomenon of occurrence of residual output also occurs. It has been recognized that such problems may occur, and the solution to the problems has been sought by the prior invention described in the specification of the prior patent application (Japanese Patent Application Number 2003-058487) (hereinafter referred as “the prior invention”).
One of the embodiments of the radiation detector in the prior invention is shown in FIG. 5 and FIG. 6. FIG. 5 is a simplified diagram of the principal part of the radiation detector of the prior invention, and FIG. 6 explains the actions of the principal part of the radiation detector of the prior invention. The radiation detector of the prior invention includes a radiation converting layer 51, an active matrix substrate 53, and a light source 55 under the active matrix substrate 53, wherein the light source applies light to the radiation converting layer 51. Furthermore, a carrier collecting electrode 57 that is divided into multiple pieces is disposed on the bottom surface of the radiation converting layer 57.
In the radiation detector composed in this way, the light source 55 applies light before a radiation incidence. As shown in FIG. 6, the electric charge generated by the light stays in regions A in the space between the carrier collecting electrodes 57. Therefore, the lines of electric force in the radiation converting layer are distorted as shown by the arrows with the dotted lines.
When incidence of radiation occurs while the light source 55 applies light, no further accumulation of electric charge occurs in the spaces between the carrier collecting electrodes 57. Therefore, the conditions of the lines of electric force and the effective sensitive area does not change and fluctuations in the detection sensitivity do not occur. When the application of the light from the light source 55 is continued after incidence of radiation stops, the electric charges accumulating in the spaces between the carrier collecting electrodes 57 are not taken out gradually and residual output does not occur. Therefore, the radiation information can be converted into the electric charge information properly without fluctuations in the radiation sensitivity.
Here, an embodiment of the light source 55 may be a transparent plate in which a light emitting diode is mounted horizontally. The light source is not restricted to a light emitting diode. However, any kind of light source selected requires a power supply and an ordinary switching type power supply is suitable for this purpose because the switching type power supply is small, light weight and high capacity.
Furthermore, any light source requires a suitable lighting circuit. For example, the light emitting diode described above requires a current limiting circuit and a cold cathode tube or the like requires a voltage boosting circuit or the like involving an inverter.
The prior invention with such a composition has turned out to have following problems.
The electric power supplied to the light source 55 contains many switching pulses, and the light source 55 has a lighting circuit. Therefore, the noise caused by switching pulses and the lighting circuit is emitted from the light source 55. Electric charge information is generated by the emitted noise at the radiation converting layer 51 or the active matrix substrate 53. Consequently the radiation detector of the prior invention detects not only the electric charge information converted from the radiation information but also the electric charge information from such noise (noise component). Since the charge information converted from radiation itself is weak, the influence of the noise component overlapping on the charge information converted from the radiation is significant.